Aligning the Ionic Nano-Channels in Nafion Membrane

Nafion membrane is the most used proton exchange membrane in PEM fuel cells and PEM water electrolysis, because of its high ionic conductivity and excellent chemical, thermal and mechanical properties. It is known that Nafion, as well as other similar proton exchange membranes, has conductive channels consisted of sulfonate ionic nano-clusters randomly distributed in nonconductive polymer matrix. If the ionic conductive channels in the membrane can be aligned, for instance by the mechanism of polar-polar interaction under electric field, the ionic conductivity of the membrane should increase, which could lead to significant performance improvement in fuel cell and water electrolysis. However, the nano-structure alignment in the Nafion membrane proved to be not straightforward. Although the alignment of ionomer phase in blend polymer membranes was achieved by exerting electric field, the addition of a nonconductive polymer led to decreased conductivity. Electric field was shown to be not effective if casting Nafion membrane with usual Nafion solutions. Reported improvement by applying electric field to solid Nafion membrane was not repeatable. In this presentation, we report our recent finding that electric field become effective in aligning the conductive channels in Nafion membrane when using a specific binary organic solvent to make Nafion casting solution. The trans-plane proton conductivity of the electrocasting Nafion membrane increases by 23% as compared to that of non-electrocasting membrane. Interestingly, the binary solvent casting membrane showed conductivity increase by 11% even without exerting electric field. Properties of the binary solvent membrane casting solution and other features of the electrocasting Nafion membrane will also be presented. Factors affecting the alignment of ionic channels in Nafion membrane will be discussed.